1. Field of Invention
The present invention relates to secure or cryptographic communication.
2. Description of Prior Art
Prior crytographic communication systems have used pseudo-random code or noise generators to prevent deciphering of the messages. Certain prior art, as in U.S. Pat. Nos. 3,659,046; 3,852,534; 3,893,031; and 3,691,472 have utilized a single such generator to encrypt the data to be transmitted.
Another pseudorandom cryptographic communication system, disclosed in U.S. Pat. No. 3,808,536, used one or more pseudo-random sequence generators to encrypt one or more characteristics of an analog signal, such as zero-crossings and envelope of the analog signal. Another pseudo-random code system, that of U.S. Pat. No. 3,773,977, utilized the pseudo-random codes to define and reallocate the time occurrence of various portions of the signal to be sent for encrypting purposes. Still another secure communication system, that of U.S. Pat. No. 3,614,316 transmitted two identical pseudo-random codes, one of which was delayed by an amount corresponding to the data content of the message to be sent. The other code was then summed with the delayed code prior to transmission.
Other types of secure communication systems existed, as in U.S. Pat. Nos. 3,681,708 and 4,011,408, but did not utilize pseudo-random codes.
One of the problems with pseudo-random code communication systems is that after a certain time, the code repeats itself, enabling the code to be broken or deciphered. Another problem in pseudo-random codes is that of synchronization. If a synchronizing code was transmitted in part of the message, the presence of such a repeatedly occurring synchronizing code afforded another way to decipher the code.